This disclosure is directed to a shock sub to be installed in a drill string. In drilling an oil well, the drill string is assembled from the requisite number of drill pipe attached to a series of heavy wall drill pipe often called drill collars and terminates at a drill bit. The drill bit rides on the bottom of the hole, and, during rotation, it chatters as it bites into the formation being drilled. This continual chatter has the form of shock impacted vertical motion, thereby imparting vibration to the entire drill string. Since the drill string is formed of metal pipe without cushioning at the joints, the vibration is coupled through the entire drill string. The vibration is a source of wear and tear, particularly at the tool joints and drill bit where the various tubular members are threaded together.
It is difficult to eliminate all vibration because the drill string, itself, provides a significant weight on the drill bit which is important to drill bit operation. It is essential that a significant weight be placed on the drill bit, and, to this end, it is not desirable to totally cushion movement of the drill bit. The present invention is an apparatus which provides a significant load on the drill bit so that drill bit operation continues in the desired manner and rate, and yet, the present apparatus additionally dampens the chatter or high frequency vibrations of the drill string. This is accomplished in an apparatus which is, in retrospect, relatively simple in construction. The apparatus incorporates a hydraulic damping system which includes an internal cavity filled with a compressible fluid which is the primary shock absorbing feature. The hydrostatic pressure tends to compress the fluid. The shock sub, however, remains elongated and does not bottom out. Elongation is helped by converting the pressure of the mud flow through the shock sub into a force impinging on a set of shoulders which tends to elongate the tool. This helps to overcome some of the reaction of the tool due to hydrostatic pressure or the weight of the drill string which tends to compress the tool and thereby helps to achieve continued operation at an intermediate point over a greater range of drilling weights and hydrostatic pressure. The intermediate point permits travel in both directions during operation.
The apparatus of this disclosure includes these features and several other features of importance wherein the shock sub is formed with pin and box connections affixed at the ends of upper and lower telescoping mandrels. The upper mandrel is formed of concentrically arranged inner and outer sleeves. They define an internal cavity which receives an upstanding sleeve from the lower mandrel. The upstanding sleeve is positioned in the axial cavity and telescopes in movement. Suitable seals prevent leakage along the telescoped upstanding sleeve. Moreover, a set of meshed splines on both mandrels permits telescoping movement while preventing relative rotation between the mandrels. The meshed splines may be cut on a helix either in the direction of rotation or against it to help to convert the torsional load to an axial load to better utilize the shock absorbing effect of the liquid spring. The sensitivity of the tool may be changed by changing the volume of fluid in the internal cavity; therefore, these changes can be accomplished by adding or deleting volume rings of a solid substance as shown in the drawing.